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  冰河的渗透实战笔记
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  B站
  <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></div><div class="nav-item"><a href="https://github.com/binghe001/BingheGuide" target="_blank" rel="noopener noreferrer" class="nav-link external">
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  <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></a></div> <!----></nav>  <ul class="sidebar-links"><li><section class="sidebar-group depth-0"><p class="sidebar-heading open"><span>JVM核心技术</span> <!----></p> <ul class="sidebar-links sidebar-group-items"><li><a href="/md/core/jvm/2022-04-18-001-JVM调优的几种场景.html" class="sidebar-link">JVM调优的几种场景</a></li><li><a href="/md/core/jvm/2022-04-18-002-类的编译.html" class="active sidebar-link">类的编译、编译优化、常量这地、泛型擦除、条件编译</a></li><li><a href="/md/core/jvm/2022-04-18-003-类的加载过程.html" class="sidebar-link">类的加载过程</a></li><li><a href="/md/core/jvm/2022-04-18-004-JVM内存空间.html" class="sidebar-link">JVM内存空间</a></li><li><a href="/md/core/jvm/2022-04-18-005-JVM堆内存分配.html" class="sidebar-link">JVM堆内存分配机制</a></li><li><a href="/md/core/jvm/2022-04-18-006-JVM垃圾回收机制.html" class="sidebar-link">JVM垃圾回收机制</a></li><li><a href="/md/core/jvm/2022-04-18-007-JVM垃圾回收算法.html" class="sidebar-link">JVM垃圾回收算法</a></li><li><a href="/md/core/jvm/2022-04-18-008-JVM-CMS垃圾收集器.html" class="sidebar-link">CMS垃圾收集器</a></li><li><a href="/md/core/jvm/2022-04-18-009-JVM -G1收集器-Region-停顿时间模型-垃圾回收.html" class="sidebar-link">G1收集器、Region、停顿时间模型、垃圾回收</a></li><li><a href="/md/core/jvm/2022-04-18-010-JVM内存布局.html" class="sidebar-link">JVM 内存布局</a></li></ul></section></li></ul> </aside> <div><main class="page"> <div class="theme-default-content content__default"><h1 id="类的编译、编译优化、常量这地、泛型擦除、条件编译"><a href="#类的编译、编译优化、常量这地、泛型擦除、条件编译" class="header-anchor">#</a> 类的编译、编译优化、常量这地、泛型擦除、条件编译</h1> <p><strong>大家好，我是冰河~~</strong></p> <p>今天，我们一起研究下JVM中类的编译问题，好了，不多说了，进入今天的正题。</p> <h2 id="编译"><a href="#编译" class="header-anchor">#</a> 编译</h2> <p>我们都知道，当我们编写完代码后，可以用javac命令或者开发工具，比如eclipse、idea等，把java文件编译成class文件，java虚拟机才可以执行。下图是WinHex打开class文件16进制的字节码。</p> <p><img alt="" data-src="https://segmentfault.com/img/bVcKG5X" loading="lazy" class="lazy"></p> <p>每个class文件的前面四个字节成为魔数，它的唯一作用就是确定这个文件是否能被虚拟机接受的文件，这个魔数值为0xCSAFEBABE（咖啡宝贝）。第5、6字节是次版本号，第7、8字节是主版本号。从上图可以看到，我的主版本号是16进制的34，也就是10进制的52，52对应的JDK版本是1.8。</p> <p>后面的16进制在这里不做过多的讲解，可以根据Java虚拟机规范的约定，以无符号数和表两种数据类型进行解析。</p> <p>java文件编译成class文件的时候，先根据词法和语法分析获得了程序代码的抽象语法树表示，填充符号表，然后根据语义分析看程序是否符合逻辑，比如变量在使用前是否已经声明，变量是否正确赋值等，最后就是把前面步骤生成的信息（比如语法树），转换成字节码写到磁盘中，当然编译器还做了少量的代码添加（程序中没有构造函数，会添加无参构造函数的操作是在填充符号表完成的，不是这里完成)和转换工作。</p> <h2 id="编译优化"><a href="#编译优化" class="header-anchor">#</a> 编译优化</h2> <h3 id="常量折叠"><a href="#常量折叠" class="header-anchor">#</a> 常量折叠</h3> <p>请看下面的例子：</p> <div class="language-typescript line-numbers-mode"><pre class="language-typescript"><code><span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">HelloWord</span> <span class="token punctuation">{</span>
     <span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">main</span><span class="token punctuation">(</span>String<span class="token punctuation">[</span><span class="token punctuation">]</span> args<span class="token punctuation">)</span> <span class="token punctuation">{</span>
           System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">&quot;Hello,&quot;</span> <span class="token operator">+</span> <span class="token string">&quot; World&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
     <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>如果我把上面的代码，改成这样的，那么我在运行期的时候，会加重虚拟机的负担吗？答案是否定的。编译器除了检查外，还对常量进行了折叠，也就是说，上面的Hello World会在编译的时候自动拼接，如果是int型的，比如i = 1 + 2，等价于 i = 3，并不会增加程序在运行期哪怕一个CPU指令的运算量。</p> <p>我们看看下面这个，如果是str += “c”呢，会不会折叠呢？</p> <div class="language-kotlin line-numbers-mode"><pre class="language-kotlin"><code><span class="token keyword">public</span> void <span class="token keyword">fun</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
    String str <span class="token operator">=</span> <span class="token string-literal singleline"><span class="token string">&quot;a&quot;</span></span> <span class="token operator">+</span> <span class="token string-literal singleline"><span class="token string">&quot;b&quot;</span></span><span class="token punctuation">;</span>
    str <span class="token operator">+=</span> <span class="token string-literal singleline"><span class="token string">&quot;c&quot;</span></span><span class="token punctuation">;</span>
    System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>str<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br></div></div><p>我们从下图的ASTView可以看出，标记1和标记2是不一样的类型，一个是变量赋值，第二个是表达式，在标记3可以看到，变量赋值的时候，自动把a和b拼接起来。</p> <p><img alt="" data-src="https://segmentfault.com/img/bVcKG6o" loading="lazy" class="lazy"></p> <h3 id="泛型的擦除"><a href="#泛型的擦除" class="header-anchor">#</a> 泛型的擦除</h3> <p>看下面的例子：</p> <div class="language-typescript line-numbers-mode"><pre class="language-typescript"><code><span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">main</span><span class="token punctuation">(</span>String<span class="token punctuation">[</span><span class="token punctuation">]</span> args<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    List<span class="token operator">&lt;</span>String<span class="token operator">&gt;</span> list1 <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ArrayList<span class="token operator">&lt;</span><span class="token operator">&gt;</span></span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    List<span class="token operator">&lt;</span>Map<span class="token operator">&lt;</span>String<span class="token punctuation">,</span> Integer<span class="token operator">&gt;&gt;</span> list2 <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ArrayList<span class="token operator">&lt;</span><span class="token operator">&gt;</span></span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>list1<span class="token punctuation">)</span><span class="token punctuation">;</span>
    System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>list2<span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><p>反编译后，如下，居然泛型还在，其实这个出乎我的意料之外，两年前给公司分享虚拟机的时候，我的ppt里还记录着泛型的擦除的案例，反编译后确实的没有泛型的。</p> <div class="language-typescript line-numbers-mode"><pre class="language-typescript"><code>  <span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">main</span><span class="token punctuation">(</span>String<span class="token punctuation">[</span><span class="token punctuation">]</span> args<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    List<span class="token operator">&lt;</span>String<span class="token operator">&gt;</span> list1 <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ArrayList<span class="token operator">&lt;</span>String<span class="token operator">&gt;</span></span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    List<span class="token operator">&lt;</span>Map<span class="token operator">&lt;</span>String<span class="token punctuation">,</span> Integer<span class="token operator">&gt;&gt;</span> list2 <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">ArrayList<span class="token operator">&lt;</span>Map<span class="token operator">&lt;</span>String<span class="token punctuation">,</span> Integer<span class="token operator">&gt;&gt;</span></span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>list1<span class="token punctuation">)</span><span class="token punctuation">;</span>
    System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>list2<span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br></div></div><p>于是我又换了另外一个反编译器，反编译如下，这次确实没有泛型信息。</p> <div class="language-csharp line-numbers-mode"><pre class="language-csharp"><code>  <span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token return-type class-name"><span class="token keyword">void</span></span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token class-name">String<span class="token punctuation">[</span><span class="token punctuation">]</span></span> args<span class="token punctuation">)</span>
  <span class="token punctuation">{</span>
    <span class="token class-name">List</span> list1 <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token constructor-invocation class-name">ArrayList</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token class-name">List</span> list2 <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token constructor-invocation class-name">ArrayList</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    System<span class="token punctuation">.</span><span class="token keyword">out</span><span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>list1<span class="token punctuation">)</span><span class="token punctuation">;</span>
    System<span class="token punctuation">.</span><span class="token keyword">out</span><span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>list2<span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><p>用java自带的命令<code>javap -verbose</code>进行反编译，结果如下，可以看到泛型信息在只有调试用的LocalVariableTypeTable（LVTT）里，某些反编译器应该根据LVTT来展示泛型信息。</p> <div class="language-awk line-numbers-mode"><pre class="language-awk"><code>LocalVariableTable<span class="token operator">:</span>
Start  Length  Slot  Name   Signature
    <span class="token number">0</span>      <span class="token number">31</span>     <span class="token number">0</span>  args   <span class="token punctuation">[</span>Ljava<span class="token operator">/</span>lang<span class="token operator">/</span>String<span class="token punctuation">;</span>
    <span class="token number">8</span>      <span class="token number">23</span>     <span class="token number">1</span> list1   Ljava<span class="token operator">/</span>util<span class="token operator">/</span>List<span class="token punctuation">;</span>
   <span class="token number">16</span>      <span class="token number">15</span>     <span class="token number">2</span> list2   Ljava<span class="token operator">/</span>util<span class="token operator">/</span>List<span class="token punctuation">;</span>
LocalVariableTypeTable<span class="token operator">:</span>
Start  Length  Slot  Name   Signature
    <span class="token number">8</span>      <span class="token number">23</span>     <span class="token number">1</span> list1   Ljava<span class="token operator">/</span>util<span class="token operator">/</span>List<span class="token operator">&lt;</span>Ljava<span class="token operator">/</span>lang<span class="token operator">/</span>String<span class="token punctuation">;</span><span class="token operator">&gt;</span><span class="token punctuation">;</span>
   <span class="token number">16</span>      <span class="token number">15</span>     <span class="token number">2</span> list2   Ljava<span class="token operator">/</span>util<span class="token operator">/</span>List<span class="token operator">&lt;</span>Ljava<span class="token operator">/</span>util<span class="token operator">/</span>Map<span class="token operator">&lt;</span>Ljava<span class="token operator">/</span>lang<span class="token operator">/</span>String<span class="token punctuation">;</span>Ljava<span class="token operator">/</span>lang<span class="token operator">/</span>Integer<span class="token punctuation">;</span><span class="token operator">&gt;</span><span class="token punctuation">;</span><span class="token operator">&gt;</span><span class="token punctuation">;</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br></div></div><p>因为泛型在编译期就会被擦除，所以以下的重载，是编译不过去的，因为擦除后，他们其实</p> <div class="language-kotlin line-numbers-mode"><pre class="language-kotlin"><code><span class="token keyword">public</span> static String <span class="token keyword">fun</span><span class="token punctuation">(</span>List<span class="token operator">&lt;</span>String<span class="token operator">&gt;</span> list<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">return</span> <span class="token keyword">null</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>

<span class="token keyword">public</span> static Integer <span class="token keyword">fun</span><span class="token punctuation">(</span>List<span class="token operator">&lt;</span>Integer<span class="token operator">&gt;</span> list<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token keyword">return</span> <span class="token keyword">null</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br></div></div><h3 id="自动装箱、拆箱与循环遍历"><a href="#自动装箱、拆箱与循环遍历" class="header-anchor">#</a> 自动装箱、拆箱与循环遍历</h3> <p>这部分代码包括自动装箱、拆箱与循环遍历。</p> <div class="language-fortran line-numbers-mode"><pre class="language-fortran"><code><span class="token keyword">public</span> void fun<span class="token punctuation">(</span><span class="token punctuation">)</span> {
    List<span class="token operator">&lt;</span><span class="token keyword">Integer</span><span class="token operator">&gt;</span> list <span class="token operator">=</span> Arrays.asList<span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">,</span> <span class="token number">2</span><span class="token punctuation">,</span> <span class="token number">3</span><span class="token punctuation">,</span> <span class="token number">4</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    int sum <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>
    for <span class="token punctuation">(</span>int i <span class="token punctuation">:</span> list<span class="token punctuation">)</span> {
        sum <span class="token operator">+</span><span class="token operator">=</span> i<span class="token punctuation">;</span>
    }
    System.<span class="token keyword">out</span>.println<span class="token punctuation">(</span>sum<span class="token punctuation">)</span><span class="token punctuation">;</span>
}
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br></div></div><p>反编译后如下，首先把int装箱变成Integer，然后计算的时候，再从Integer拆卸变成int。另外一个就是把for编译成Iterator迭代器。</p> <div class="language-fortran line-numbers-mode"><pre class="language-fortran"><code><span class="token keyword">public</span> void fun<span class="token punctuation">(</span><span class="token punctuation">)</span>
{
    List list <span class="token operator">=</span> Arrays.asList<span class="token punctuation">(</span>new <span class="token keyword">Integer</span>[] { <span class="token keyword">Integer</span>.valueOf<span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">)</span><span class="token punctuation">,</span> <span class="token keyword">Integer</span>.valueOf<span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">)</span><span class="token punctuation">,</span> <span class="token keyword">Integer</span>.valueOf<span class="token punctuation">(</span><span class="token number">3</span><span class="token punctuation">)</span><span class="token punctuation">,</span> <span class="token keyword">Integer</span>.valueOf<span class="token punctuation">(</span><span class="token number">4</span><span class="token punctuation">)</span> }<span class="token punctuation">)</span><span class="token punctuation">;</span>
    int sum <span class="token operator">=</span> <span class="token number">0</span><span class="token punctuation">;</span>
    for <span class="token punctuation">(</span>Iterator localIterator <span class="token operator">=</span> list.iterator<span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> localIterator.hasNext<span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span> <span class="token punctuation">)</span> { 
        int i <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token punctuation">(</span><span class="token keyword">Integer</span><span class="token punctuation">)</span>localIterator.next<span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span>.intValue<span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        sum <span class="token operator">+</span><span class="token operator">=</span> i<span class="token punctuation">;</span>
    }
    System.<span class="token keyword">out</span>.println<span class="token punctuation">(</span>sum<span class="token punctuation">)</span><span class="token punctuation">;</span>
}
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br></div></div><p>另外看看下面的例子</p> <div class="language-pgsql line-numbers-mode"><pre class="language-text"><code>public static void main(String[] args) {
    Integer a = 1;
    Integer b = 2;
    Integer c = 3;
    Integer d = 4;
    Integer e = 321;
    Integer f = 321;
    Long g = 3L;
    System.out.println(c == d);
    System.out.println(e == f);
    System.out.println(c == (a + b));
    System.out.println(c.equals(a + b));
    System.out.println(g == (a + b));
    System.out.println(g.equals(a + b));
}
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br></div></div><p>反编译后</p> <div class="language-reasonml line-numbers-mode"><pre class="language-text"><code>  public static void main(String[] args)
  {
    Integer a = Integer.valueOf(1);
    Integer b = Integer.valueOf(2);
    Integer c = Integer.valueOf(3);
    Integer d = Integer.valueOf(4);
    Integer e = Integer.valueOf(321);
    Integer f = Integer.valueOf(321);
    Long g = Long.valueOf(3L);
    //为false，Integer对象，比较地址，两个地址不一样
    System.out.println(c == d);
    //为false，因为两个地址不一样
    System.out.println(e == f);
    //为true，都变成int，比较值
    System.out.println(c.intValue() == a.intValue() + b.intValue());
   //为true，equals方法比较int System.out.println(c.equals(Integer.valueOf(a.intValue() + b.intValue())));
   //为true，比较值
    System.out.println(g.longValue() == a.intValue() + b.intValue());
    //为false，类型不一样
    System.out.println(g.equals(Integer.valueOf(a.intValue() + b.intValue())));
  }
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br><span class="line-number">13</span><br><span class="line-number">14</span><br><span class="line-number">15</span><br><span class="line-number">16</span><br><span class="line-number">17</span><br><span class="line-number">18</span><br><span class="line-number">19</span><br><span class="line-number">20</span><br><span class="line-number">21</span><br></div></div><h3 id="条件编译"><a href="#条件编译" class="header-anchor">#</a> 条件编译</h3> <p>下面这个包括没有使用的变量a，if语句判断，while语句。</p> <div class="language-nim line-numbers-mode"><pre class="language-nim"><code>public <span class="token keyword">static</span> void <span class="token function">main</span><span class="token punctuation">(</span>String<span class="token punctuation">[</span><span class="token punctuation">]</span> args<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    int a;
    <span class="token function">if</span><span class="token punctuation">(</span>true<span class="token punctuation">)</span><span class="token punctuation">{</span>
        System<span class="token operator">.</span><span class="token keyword">out</span><span class="token operator">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">&quot;Hello&quot;</span><span class="token punctuation">)</span>;
    <span class="token punctuation">}</span><span class="token keyword">else</span><span class="token punctuation">{</span>
        System<span class="token operator">.</span><span class="token keyword">out</span><span class="token operator">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">&quot;World&quot;</span><span class="token punctuation">)</span>;
    <span class="token punctuation">}</span>
   <span class="token operator">/*</span> <span class="token function">while</span><span class="token punctuation">(</span>false<span class="token punctuation">)</span><span class="token punctuation">{</span>
        <span class="token operator">//</span> Unreachable statement
        System<span class="token operator">.</span><span class="token keyword">out</span><span class="token operator">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">&quot;while&quot;</span><span class="token punctuation">)</span>;
    <span class="token punctuation">}</span><span class="token operator">*/</span>
<span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br><span class="line-number">5</span><br><span class="line-number">6</span><br><span class="line-number">7</span><br><span class="line-number">8</span><br><span class="line-number">9</span><br><span class="line-number">10</span><br><span class="line-number">11</span><br><span class="line-number">12</span><br></div></div><p>反编译后，没有使用的变量a并没有被编译，Dead code，也就是为false的部分也没有被编译，另外while那个，由于恒为false，编译器报Unreachable statement。</p> <div class="language-typescript line-numbers-mode"><pre class="language-typescript"><code>  <span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">main</span><span class="token punctuation">(</span>String<span class="token punctuation">[</span><span class="token punctuation">]</span> args<span class="token punctuation">)</span>
  <span class="token punctuation">{</span>
    System<span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">&quot;Hello&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>
</code></pre> <div class="line-numbers-wrapper"><span class="line-number">1</span><br><span class="line-number">2</span><br><span class="line-number">3</span><br><span class="line-number">4</span><br></div></div><p><strong>好了，今天就到这儿吧，我是冰河，我们下期见~~</strong></p> <h2 id="写在最后"><a href="#写在最后" class="header-anchor">#</a> 写在最后</h2> <blockquote><p>如果你觉得冰河写的还不错，请微信搜索并关注「 <strong>冰河技术</strong> 」微信公众号，跟冰河学习高并发、分布式、微服务、大数据、互联网和云原生技术，「 <strong>冰河技术</strong> 」微信公众号更新了大量技术专题，每一篇技术文章干货满满！不少读者已经通过阅读「 <strong>冰河技术</strong> 」微信公众号文章，吊打面试官，成功跳槽到大厂；也有不少读者实现了技术上的飞跃，成为公司的技术骨干！如果你也想像他们一样提升自己的能力，实现技术能力的飞跃，进大厂，升职加薪，那就关注「 <strong>冰河技术</strong> 」微信公众号吧，每天更新超硬核技术干货，让你对如何提升技术能力不再迷茫！</p></blockquote> <p><img alt="" data-src="https://img-blog.csdnimg.cn/20200906013715889.png" loading="lazy" class="lazy"></p></div> <footer class="page-edit"><div class="edit-link"><a href="https://github.com/binghe001/BingheGuide/edit/master/docs/md/core/jvm/2022-04-18-002-类的编译.md" target="_blank" rel="noopener noreferrer">在 GitHub 上编辑此页</a> <span><svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" focusable="false" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg> <span class="sr-only">(opens new window)</span></span></div> <div class="last-updated"><span class="prefix">上次更新: </span> <span class="time">2022/5/23</span></div></footer> <div class="page-nav"><p class="inner"><span class="prev">
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